How do I keep language selection separate for individual apps?

Previously I could use language X in 'App A' while using language Y and 'App b' in parallel.

This allowed me to command+tab between apps without needing to switch between languages.

I've apparently hit a weird key combo and now whichever language I've selected is activated across the board.

Does anybody know how do I switch back to the default mode?

Super frustrating and time consuming after being used to that separate language selection mode (which was one of the coolest features to me, back when I switched from Windows, tbh!).

Thanks in advance!!!